This is one of the reasons why most fossil shark remains only consist of teeth. Unfortunately for palaeontologists (people who study fossils) cartilage normally decays before it can be fossiled. This is useful for the shark, because unlike bony fishes they do not have an air-filled bladder to keep them bouyant in the water. This is unusual as sharks skeletons are mostly made of cartilage, a gristle-like material that has the advantage of being lighter than bone. Despite these looks it is in fact the bone from a fossilised Jurassic shark. It is clearly a fossilised bone, but what might it be? It looks a bit like a jawbone, along the right edge are curved and pointed hooks that could easily be teeth. The study of modern day creatures is an invaluable way of gaining an insight into what ancient creatures may have looked like and how they could have behaved and interacted together.ДЄ good example of this is the strange fossil shown in the photo. In this way nautilus is jetted propelled through the water. When the water is squirted back out it pushes the animal backward. This is used to draw water to the gills, where the oxygen is extracted. Below the tentacles is a large open tube called the siphon. Like an octopus it has a sharp beak to tear and cut its food. In the middle of the tentacles is the animals mouth.

Nautilus catches its food with a mass of tentacles, just like a squid. This picture shows a modern nautilus shell that has been section in the same way as the fossil ammonite, the similarity between the two is readily seen. The video below shows how a nautilus swims.Д¬lick here for information on our Fossil Walks. This allows it to move up and down in the water, just like a submarine. The living nautilus uses the siphuncle to adjust the proportion of water and gas in the chambers of its shell. This chamber makes up about half the final whorl. The fleshy body of the nautilus is only in the final chamber of the shell. A spiral tube called the siphuncle runs through all the chambers to the centre of the shell. In a living nautilus the chambered sections of the shell contain nitrogen gas and water. As nautilus is a living creature we can look at its soft parts and see how it uses the shell. The nautilus is related to creatures such as the octopus and squid but it has a spirally coiled shell just like the ammonite. The closest comparison to an ammonite we have is a living sea creature called the nautilus. This is very different to a snail shell which is a simple open spiral inside. You can see the inside is divided into many small chambers. The accompanying picture shows an ammonite that has been cut in half and polished. Externally the shell looks like that of a snail, but internally it is very different.

Admire this elegant ammonite in Bridport Museum.Most people are familiar with the spirally coiled ammonite shell.

In England, particularly around Whitby, people once thought that ammonites were coiled snakes that had been turned to rock.

Some ammonite shells have bite marks from these marine monsters.

Ammonites were food for creatures such as ichthyosaurs and pliosaurs.

We know lots about ammonites because their closest living relative, the nautilus, still survives today in the Pacific and Indian Oceans.

Only the shells of ammonites have ever been found as fossils.

When the animal was alive, it could fill the chambers with gas and water to move up or down.

Ammonites have a spiral shell divided into chambers.

So they jet-propelled themselves вЂ“ backwards!

They scooted through the warm, shallow seas by squirting jets of water from their bodies.

Ammonites are extinct marine creatures вЂ“ and some of the most commonly found fossils.ArtistвЂ™s impression of a living ammonite Ammonites вЂ“ the ultimate jet-propelled shells!

Ammonite creature